A proton exchange membrane fuel cell (PEMFC) is a device which directly uses hydrogen (H2) and oxygen (O2) in an electrochemical reaction to generate electricity. The PEMFC has low operating temperature, short turn-on time, high energy density, low pollution, and wide applications, and is therefore a technique widely researched and promoted all over the world.
A typical PEMFC mainly consists of a proton exchange membrane (PEM), a catalyst layer, a gas diffusion layer (GDL), and a bipolar plate. The proton exchange membrane (PEM) is a solid-state polymeric membrane, such as Nafion® membranes from DuPont®, Aciplex® membranes from Asashi Chemical, BAM® (Ballard Advanced Material) membranes from Ballard, and Gore-Select® membranes from Gore. The PEM is used in a PEMFC mainly to isolate the reactant gas at the cathode from that at the anode, and to isolate electrons. The PEM conducts only water molecules (H2O) and hydrogen ions (H+). Therefore, this type of polymeric membrane is a gas impermeable membrane, which conducts hydrogen ions (H+) but not electrons (e−). When the hydrogen ions (H+) are conducted via this type of polymeric membrane, they must be carried by water molecules. Therefore, the higher the moisture content of the polymeric membrane is, the better the hydrogen ions (H+) conducting is. Thus, it is important to increase the moisture content of the polymeric membrane to obtain better hydrogen ions (H+) conducting, and accordingly, to maintain the PEMFC in good performance.
The methods and designs for humidifying a reactant gas for the fuel cell may be generally divided into two types, namely, internal and external humidification, that have their respective advantages and drawbacks. Regarding the external humidification, an external humidifier has to be provided outside the fuel cell. The external humidifier disadvantageously occupies additional space and requires additional power supply to a heater provided therein for increasing the temperature and humidity of the reactant gas. However, the external humidifier also has many advantages, such as providing stable humidifying amount, capable of controlling and regulating humidifying amount, capable of handling a relatively large amount of gas humidifying, and easy to maintain and repair. Regarding the internal humidification, humidifying mechanisms are internally added to a fuel cell. The internal humidifying mechanisms have the advantages of having small volume without occupying too much space, omitting additional humidifier and heater to save the cost therefor, and directly utilizing recycled waste heat or water produced by the fuel cell itself. However, the internal humidifying mechanisms also have some drawbacks, such as involving complicate pipeline design and complicate connection to the fuel cell, uneasy to control and regulate the humidifying amount, uneasy to get saturated humidified gas when the load is high, and uneasy to maintain and repair.
In recent years, many reactant gas humidifying designs for fuel cell systems have been made or improved. U.S. Pat. Nos. 5,482,680 and 5,527,363 disclose a fuel cell stack having a humidifying section. This type of fuel cell stack has disadvantageously largely increased volume and weight, and the flow field design for the fuel, oxidizer, and water in the fuel cell stack is very complicate. U.S. Pat. No. 6,406,807 teaches the forming of water spray holes on ribs or lands between the gas passages on a carbon plate, so as to humidify the PEM directly. U.S. Pat. No. 6,403,249 discloses a battery with a membrane type humidifying section to directly humidify a reactant gas. U.S. Pat. No. 6,207,312 discloses a self-humidifying design, in which an interdigitated flow field and a membrane type humidifying section provided on a carbon plate are adopted. U.S. Pat. No. 6,066,408 teaches the addition of a wick in the gar flow field, and the use of water adding holes to supplement the water content of the wick. U.S. Pat. No. 5,998,054 discloses a humidifying design in which water is sprayed at a front end of every gas flow field on the carbon plate. U.S. Pat. No. 5,952,119 discloses a gas diffusion layer formed from a carbon fabric, on which hydrophilic fine threads are sewed at regular intervals, so that supplemented water is distributed over the PEM via the hydrophilic fine threads. And, U.S. Pat. No. 5,965,288 teaches an external water-permeable membrane humidifier to humidify a reactant gas.